How I Wrote a Modern C++ Library in Rust

Since version 56, Firefox has had a new character encoding conversion library called encoding_rs. It is written in Rust and replaced the old C++ character encoding conversion library called uconv that dated from early 1999. Initially, all the callers of the character encoding conversion library were C++ code, so the new library, despite being written in Rust, needed to feel usable when used from C++ code. In fact, the library appears to C++ callers as a modern C++ library. Here are the patterns that I used to accomplish that.

gsl:: above refers to the Guidelines Support Library, which provides things that the Core Guidelines expect to have available but that are not (yet) in the C++ standard library.

C++ Library in Rust?

By writing a C++ library “in Rust” I mean that the bulk of the library is actually a library written in Rust, but the interface provided to C++ callers makes it look and feel like a real C++ library as far as the C++ callers can tell.

Both C++ and Rust Have C Interop

C++ has a very complex ABI, and the Rust ABI is not frozen. However, both C++ and Rust support functions that use the C ABI. Therefore, interoperability between C++ and Rust involves writing things in such a way that C++ sees Rust code as C code and Rust sees C++ code as C code.

Simplifying Factors

This write-up should not be considered a comprehensive guide to exposing Rust code to C++. The interface to encoding_rs is simple enough that it lacks some complexities that one could expect from the general case of interoperability between the two languages. However, the factors that simplify the C++ exposure of encoding_rs can be taken as a guide to simplifications that one should seek to achieve in the interest of easy cross-language interoperability when designing libraries. Specifically:

encoding_rs never calls out to C++: The cross-language calls are unidirectional.

encoding_rs does not hold references to C++ objects after a call returns: There is no need for Rust code to manage C++ memory.

encoding_rs does not present an inheritance hierarchy either in Rust or in C++: There are no vtables on either side.

The datatypes that encoding_rs operates on are very simple: Contiguous buffers of primitives (buffers of u8/uint8_t and u16/char16_t).

Only the panic=abort configuration (i.e. a Rust panic terminates the program instead of unwinding the stack) is supported and the code presented here is only correct if that option is used. The code presented here does not try to prevent Rust panics from unwinding across the FFI, and letting a panic unwind across the FFI is Undefined Behavior. (Update: The default behavior is on track to being changed.)

A Very Quick Look at the API

To get an idea about the Rust API under discussion, let’s take a high-level look. The library has three public structs: Encoding, Decoder and Encoder. From the point of view of the library user, these structs are used like traits, superclasses or interfaces in the sense that they provide a uniform interface to various concrete encodings, but technically they are indeed structs. Instances of Encoding are statically allocated. Decoder and Encoder encapsulate the state of a streaming conversion and are allocated at run-time.

A reference to an Encoding, that is &'static Encoding, can be obtained either from label (textual identification extracted from protocol text) or by a named static. The Encoding can then be used as a factor for a Decoder, which is stack-allocated.

The Process

0. Designing for FFI-friendliness

Some of the simplifying factors arise from the problem domain itself. Others are a matter of choice.

A character encoding library could reasonably present traits (similar to abstract superclasses with no fields in C++) for each of the concepts of an encoding, a decoder and an encoder. Instead, encoding_rs has structs for these that internally match on an enum for dispatch instead of relying on a vtable.

The primary motivation for this wasn’t as much eliminating vtables per se but to make the hierarchy intentionally unextensible. This reflects a philosophy that adding character encodings is not something that programmers should do. Instead, programs should use UTF-8 for interchange, and programs should support legacy encodings only to the extent necessary for compatibility with existing content. The non-extensibility of the hierarchy provides stronger type-safety. If you have an Encoding from encoding_rs, you can trust that it doesn’t exhibit characteristics that aren’t exhibited by the encodings defined in the Encoding Standard. That is, you can trust that it won’t behave like UTF-7 or EBCDIC.

Additionally, by dispatching on an enum, a decoder for one encoding can internally morph into a decoder for another encoding in response to BOM sniffing.

One might argue that the Rustic way to provide encoding converters would be making them into iterator adaptors that consume an iterator of bytes and yield Unicode scalar values or vice versa. In addition to iterators being more complex to expose across the FFI, iterators make it harder to perform tricks to accelerate ASCII processing. Taking a slice to read from and a slice to write to not only makes it easier to represent things in a C API (in C terms, a Rust slice decomposes to an aligned non-null pointer and a length) but also enables ASCII acceleration by processing more than one code unit at a time making use of the observation that multiple code units fit in a single register (either an ALU register or a SIMD register).

If the Rust-native API deals only with primitives, slices and (non-trait object) structs, it is easier to map to a C API than a Rust API that deals with fancier Rust features. (In Rust, you have a trait object when type erasure happens. That is, you have a trait-typed reference that does not say the concrete struct type of the referent that implements the trait.)

1. Creating the C API

When the types involved are simple enough, the main mismatches between C and Rust are the lack of methods and multiple return values in C and the inability to transfer non-C-like structs by value.

Methods are wrapped by functions whose first argument is a pointer to the struct whose method is being wrapped.

Slice arguments become two arguments: the pointer to the start of the slice and the length of the slice.

One primitive value is returned as a function return value and the rest become out params. When the output params clearly relate to inputs of the same type, it makes sense to use in/out params.

When a Rust method returns the struct by value, the wrapper function boxes it and returns a pointer such that the Rust side forgets about the struct. Additionally, a function for freeing a given struct type by pointer is added. Such a method simply turns pointer back into a box and drops the box. The struct is opaque from the C point of view.

As a special case, the method for getting the name of an encoding, which in Rust would return &'static str is wrapped by a function that takes a pointer to writable buffer whose length must be at least the length of the longest name.

enums signaling the exhaustion of the input buffer, the output buffer becoming full or errors with detail about the error became uint32_t with constants for “input empty” and “output full” and rules for how to interpret the other error details. This isn’t ideal but works pretty well in this case.

Overflow-checking length computations are presented as saturating instead. That is, the caller has to treat SIZE_MAX as a value signaling overflow.

2. Re-Creating the Rust API in C++ over the C API

Even an idiomatic C API doesn’t make for a modern C++ API. Fortunately, Rustic concepts like multiple return values and slices can be represented in C++, and by reinterpreting pointers returned by the C API as pointers to C++ objects, it’s possible to present the ergonomics of C++ methods.

Method Ergonomics

For each opaque struct pointer type in C, a class is defined in C++ and the C header is tweaked such that the pointer types become pointers to instances of the C++ classes from the point of view of the C++ compiler. This amounts to a reinterpret_cast of the pointers without actually writing out the reinterpret_cast.

Since the pointers don’t truly point to instances of the classes that they appear to point to but point to instances of Rust structs instead, it’s a good idea to take some precautions. No fields are declared for the classes. The default no-argument and copy constructors are deleted as is the default operator=. Additionally, there must be no virtual methods. (This last point is an important limitation that will come back to later.)

In the case of Encoding whose all instances are static, the destructor is deleted as well. In the case of the dynamically-allocated Decoder and Encoder both an empty destructor and a static void operator delete is added. (An example follows a bit later.) This enables the destruction of the fake C++ class to be routed to the right type-specific freeing function in the C API.

With that foundation in place to materialize pointers that look like pointers to C++ class instances, it’s possible to make method calls on this pointers work. (An example follows after introducing the next concept, too.)

Returning Dynamically-Allocated Objects

As noted earlier, the cases where the Rust API would return an Encoder or a Decoder by value so that the caller can place them on the stack is replaced by the FFI wrapper boxing the objects so that the C API exposes only heap-allocated objects by pointer. Also, the reinterpretation of these pointers as deleteable C++ object pointers was already covered.

That still leaves making sure that delete is actually used at an appropriate time. In modern C++, when an object can have only one legitimate owner of the time, this is accomplished by wrapping the object pointer in std::unique_ptr or mozilla::UniquePtr. The old uconv converters supported reference counting, but all the actual uses in the Gecko code base involved only one owner for each converter. Since the usage patterns of encoders and decoders are such that there is only one legitimate owner of the time, using std::unique_ptr and mozilla::UniquePtr is what the two C++ wrappers for encoding_rs do.

Let’s take a look at a factory method on Encoding that returns a Decoder. In Rust, we have a method that takes a reference to self and returns Decoder by value.

On the FFI layer, we have an explicit pointer-typed first argument that corresponds to Rust &self and C++ this (specifically, the const version of this). We allocate memory on the heap (Box::new()) and place the Decoder into the allocated memory. We then forget about the allocation (Box::into_raw) so that we can return the pointer to C without deallocating at the end of the scope. In order to be able to free the memory, we introduce a new function that puts the Box back together and assigns it into a variable that immediately goes out of scope causing the heap allocation to be freed.

How Can it Work?

A method call on non-trait-typed reference is just a plain function call with the reference to self as the first argument. On the C++ side, non-virtual method calls work the same way: A non-virtual C++ method call is really just a function call whose first argument is the this pointer.

On the FFI/C layer, we can pass the same pointer as an explicit pointer-typed first argument.

When calling ptr->Foo() where ptr is of type T*, the type of this is T* if the method is declared as void Foo() (which maps to &mut self in Rust) and const T* if the method is declared as void Foo() const (which maps to &self in Rust), so const-correctness is handled, too.

fnfoo(&self, bar: usize) -> usize

size_t foo(size_t bar) const

fnfoo(&mutself, bar: usize) -> usize

size_t foo(size_t bar)

The qualifications about “non-trait-typed” and “non-virtual” are important. For the above to work, we can’t have vtables on either side. This means no Rust trait objects and no C++ inheritance. In Rust, trait objects, i.e. trait-typed references to any struct that implements the trait, are implemented as two pointers: one to the struct instance and another to the vtable appropriate for the concrete type of the data. We need to be able to pass reference to self across the FFI as a single pointer, so there’s no place for the vtable pointer when crossing the FFI. In order to keep pointers to C++ objects as C-compatible plain pointers, C++ puts the vtable pointer on the objects themselves. Since the pointers don’t really point to C++ objects carrying vtable pointers but point to Rust objects, we must make sure not to make the C++ implementation expect to find a vtable pointer on the pointee.

As a consequence, the C++ reflector classes for the Rust structs cannot inherit from a common baseclass of a C++ framework. In the Gecko case, the reflector classes cannot inherit from nsISupports. E.g. in the context of Qt, the reflector classes wouldn’t be able to inherit from QObject.

Non-Nullable Pointers

There are methods in the Rust API that return &'static Encoding. Rust references can never be null, and it would be nice to relay this piece of information in the C++ API. It turns out that there is a C++ idiom for this: gsl::not_null and mozilla::NotNull.

Since gsl::not_null and mozilla::NotNull are just type system-level annotations that don’t change the machine representation of the underlying pointer and since from the guarantees Rust we know which pointers that we get from the FFI really never are null, it is tempting to apply the same reinterpretation trick of lying to the C++ compiler about types that we use to reinterpret pointers returned by the FFI as pointers to fieldless C++ objects with no virtual methods and to claim in a header file that the pointers that we know not to be null in the FFI return values are of the type mozilla::NotNull<const Encoding*>. Unfortunately, this doesn’t actually work because types involving templates are not allowed in the declarations of extern "C" functions in C++, so the C++ code ends up executing a branch for the null check when wrapping pointers received from the C API with gsl::not_null or mozilla::NotNull.

However, there are also declarations of static pointers to the constant encoding objects (where the pointees are defined in Rust) and it happens that C++ does allow declaring those as gsl::not_null<const Encoding*>, so that is what is done. (Thanks to Masatoshi Kimura for pointing out that this is possible.)

The statically-allocated instances of Encoding are declared in Rust like this:

In Rust, the general rule is that you use static for an unchanging memory location and const for an unchanging value. Therefore, UTF_8_INIT should be static and UTF_8 should be const: the value of the reference to the static instance is unchanging, but statically allocating a memory location for the reference is not logically necessary. Unfortunately, Rust has a rule that says that the right-hand side of const may not contain anything static and this is applied so heavily as to prohibit even references to static, in order to ensure that the right-hand side of a const declaration can be statically checked to be suitable for use within any imaginable const declaration—even one that tried to dereference the reference at compile time.

For FFI, though, we need to allocate an unchanging memory location to a pointer to UTF_8_INIT, because such memory locations work in C linkage and allow us provide a pointer-typed named thing to C. The representation of UTF_8 above is already what we need, but for Rust ergonomics, we want UTF_8 to participate in Rust’s crate namespacing. This means that from the C perspective the name gets mangled. We waste some space by statically allocating pointers again without name mangling for C usage:

A pointer type is used to make in clear that C is supposed to see a pointer (even if a Rust reference type would have the same representation). However, the Rust compiler refuses to compile a program with globally-visible pointer. Since globals are reachable from different threads, multiple threads accessing the pointee might be problem. In this case, the pointee cannot be mutated, so global visibility is fine. To tell the compiler that this is fine, we need to implement the Sync marker trait for the pointer. However, traits cannot be implemented on pointer types. As a workaround, we create a newtype for *const Encoding. A newtype has the same representation as the type it wraps, but we can implement traits on the newtype. Implementing Sync is unsafe, because we are asserting to the compiler that something is OK when the compiler does not figure it out on its own.

The pointers to the encoders and decoders are also known not to be null, since allocation failure would terminate the program, but std::unique_ptr / mozilla::UniquePtr and gsl::not_null / mozilla::NotNull cannot be combined.

Optional Values

In Rust, it’s idiomatic to use Option<T> to represent return values might either have a value or might not have a value. C++ these days provides the same thing as std::optional<T>. In Gecko, we instead have mozilla::Maybe<T>.

Rust’s Option<T> and C++’s std::optional<T> indeed are basically the same thing:

returnNone;

returnstd::nullopt;

returnSome(foo);

return foo;

is_some()

operatorbool()

has_value()

unwrap()

value()

unwrap_or(bar)

value_or(bar)

Unfortunately, though, C++ reverses the safety ergonomics. The most ergonomic way to extract the wrapped value from a std::optional<T> is via operator*(), which is unchecked and, therefore, unsafe. 😭

Multiple Return Values

While C++ lacks language-level support for multiple return values, multiple return values are possible thanks to library-level support. In the case of the standard library, the relevant library pieces are std::tuple, std::make_tuple and std::tie. In the case of Gecko, the relevant library pieces are mozilla::Tuple, mozilla::MakeTuple and mozilla::Tie.

fnfoo() -> (T, U, V)

std::tuple<T, U, V> foo()

return (a, b, c);

return {a, b, c};

let (a, b, c) = foo();

constauto [a, b, c] = foo();

letmut (a, b, c) = foo();

auto [a, b, c] = foo();

Slices

A Rust slice wraps a non-owning pointer and a length that identify a contiguous part of an array. In comparison to C:

src: &[u8]

constuint8_t* src, size_t src_len

dst: &mut [u8]

uint8_t* dst, size_t dst_len

There isn’t a corresponding thing in the C++ standard library yet (except std::string_view for read-only string slices), but it’s already part of the C++ Core Guidelines and is called a span there.

src: &[u8]

gsl::span<constuint8_t> src

dst: &mut [u8]

gsl::span<uint8_t> dst

&mut vec[..]

gsl::make_span(vec)

std::slice::from_raw_parts(ptr, len)

gsl::make_span(ptr, len)

for item in slice {}

for (auto&& item : span) {}

slice[i]

span[i]

slice.len()

span.size()

slice.as_ptr()

span.data()

GSL relies on C++14, but at the time encoding_rs landed, Gecko was stuck on C++11 thanks to Android. Since, GSL could not be used as-is in Gecko, I backported gsl::span to C++11 as mozilla::Span. The porting process was mainly a matter of ripping out constexpr keywords and using mozilla:: types and type traits in addition to or instead of standard-library ones. After Gecko moved to C++14, some of the constexpr keywords have been restored.

Once we had our own mozilla::Span anyway, it was possible to add Rust-like subspan ergonomics that are missing from gsl::span. For the case where you want a subspan from index i up to but not including index j. gsl::span has:

&slice[i..]

span.subspan(i)

&slice[..i]

span.subspan(0, i)

&slice[i..j]

span.subspan(i, j - i)😭

mozilla::Span instead has:

&slice[i..]

span.From(i)

&slice[..i]

span.To(i)

&slice[i..j]

span.FromTo(i, j)

gsl::span and Rust slices have one crucial difference in how they decompose into a pointer and a length. For zero-length gsl::span it is possible for the pointer to be nullptr. In the case of Rust slices, the pointer must always be non-null and aligned even for zero-length slices. This may look counter-intuitive at first: When the length is zero, the pointer never gets dereferenced, so why doesn’t matter whether it is null are not? It turns out that it matters for optimizing out the enum discriminant in Option-like enums. None is represented by all-zero bits, so if wrapped in Some(), a slice with null as the pointer and zero as the length would accidentally have the same representation as None. By requiring the pointer to be a potentially bogus non-null pointer, a zero-length slice inside an Option can be represented distinctly from None without a discriminant. By requiring the pointer to be aligned, further uses of the low bits of the pointer are possible when the alignment of the slice element type is greater than one.

After realizing that it’s not okay to pass the pointer obtained from C++ gsl::span::data() to Rust std::slice::from_raw_parts() as-is, it was necessary to decide where to put the replacement of nullptr with reinterpret_cast<T*>(alignof(T)). There are two candidate locations when working with actual gsl::span: In the Rust code that provides the FFI or in the C++ code that calls the FFI. When working with mozilla::Span, the code of the span implementation itself could be changed, so there are two additional candidate locations for the check: the constructor of mozilla::Span and the getter for the pointer.

Of these for candidate locations, the constructor of mozilla::Span seemed like the one where the compiler has the best opportunity to optimize out the check in some cases. That’s why I chose to put the check there. This means that in the gsl::span scenario the check had to go in the code that calls the FFI. All pointers obtained from gsl::span have to be laundered through:

Additionally, this means that since the check is not in the code that provides the FFI, the C API became slightly unidiomatic in the sense that requires C callers to avoid passing in NULL even when the length is zero. However, the C API already has many caveats about things that are Undefined Behavior, and adding yet another thing that is documented to be Undefined Behavior does seem like an idiomatic thing to do with C.

Putting it Together

Let’s look at an example of how the above features combine. First, in Rust we have a method that takes a slice and returns an optional tuple:

Since this is a static method, there is no reference to self and no corresponding pointer in the FFI function. The slice decomposes into a pointer and a length. The length becomes an in/out param that communicates the length of the slice in and the length of the BOM sublice out. The encoding becomes the return value and the encoding pointer being null communicates the Rust None case for the tuple.

Here we have to explicitly use std::make_tuple, because the implicit constructor doesn’t work when the std::tuple is nested inside std::optional.

Algebraic Types

Early on, we saw that the Rust-side streaming API can return this enum:

pubenumDecoderResult {
InputEmpty,
OutputFull,
Malformed(u8, u8),
}

C++ now has an analog for Rust enum, sort of: std::variant<Types...>. In practice, though, std::variant is so clunky that it does not make sense to use it when a Rust enum is supposed to act in a lightweight way from the point view of ergonomics.

First, the variants in std::variant aren’t named. They are identified positionally or by type. Named variants were proposed as proposed as lvariant but did not get accepted. Second, even though duplicate types are permitted, working with them is not practical. Third, there is no language-level analog for Rust’s match. A match-like mechanism was proposed as inspect() but was not accepted.

On the FFI/C layer, the information from the above enum is packed into a u32. Instead of trying to expand it to something fancier on the C++ side, the C++ API uses the same uint32_t as the C API. If the caller actually cares about extracting the two small integers in the malformed case, it’s up to the caller to do the bitwise ops to extract them from the uint32_t.

Using zero as the magic value for INPUT_EMPTY is a premature micro-optimization. On some architectures comparison with zero is cheaper than comparison with other constants, and the values representing the malformed case when decoding and the unmappable case when encoding are known not to overlap zero.

Signaling Integer Overflow

Decoder and Encoder have methods for querying worst-case output buffer size requirement. The caller provides the number of input code units and the method returns the smallest output buffer length, in code units, that guarantees that the corresponding conversion method will not return OutputFull.

E.g. when encoding from UTF-16 to UTF-8, calculating the worst case involves multiplication by three. Such a calculation can, at least in principle, result in integer overflow. In Rust, integer overflow is considered safe, because even if you allocate too short a buffer as a result of its length computation overflowing, actually accessing the buffer is bound checked, so the overall result is safe. However, buffer access is not generally bound checked in C or C++, so an integer overflow in Rust can result in memory unsafety in C or C++ if the result of the calculation that overflowed is used for deciding the size of buffers allocated and accessed by C or C++ code. In the case of encoding_rs, even when C or C++ allocates the buffer, the writing is supposed to be performed by Rust code, so it might be OK. However, to be sure, the worst-case calculations provided by encoding_rs used overflow-checking arithmetic.

In Rust, the methods whose arithmetic is overflow-checked return Option<usize>. To keep the types of the C API simple, the C API returns size_t with SIZE_MAX signaling overflow. That is, the C API effectively appears as using saturating arithmetic.

In the C++ API version that uses standard-library types, the return type is std::optional<size_t>. In Gecko, we have a wrapper for integer types that provides overflow-checking arithmetic and a validity flag. In the Gecko version of the C++ API, the return type is mozilla::CheckedInt<size_t> so that dealing with overflow signaling is uniform with the rest of Gecko code. (Aside: I find it shocking and dangerous that the C++ standard library still does not provide a wrapper similar to mozilla::CheckedInt in order to do overflow-checking integer math in a standard-supported Undefined Behavior-avoiding way.)

Recreating the Non-Streaming API

Let’s look again at the example of a non-streaming API method on Encoding:

This type inside the Option in the return type is Cow<'a, str>, which is a type that holds either an owned String or a borrowed string slice (&'a str) whose data is owned by someone else. The lifetime 'a of the borrowed string slice is the lifetime of the input slice (bytes: &'a [u8]), because in the borrow case the output is actually borrowed from the input.

Mapping this kind of return type to C poses problems. First of all, C does not provide a great way to say that we either have the owned case or we have the borrowed case. Second, C does not have a standard type for heap-allocated strings that know their length and capacity and that can reallocate their buffer when modified. Maybe this could be seen as an opportunity to create a new C type whose buffer is managed by Rust String, but then such a type would not fit together with C++ strings. Third, a borrowed string slice in C would be a raw pointer and a length and some documentation that says that the pointer is valid only as long as the input pointer is valid. There would be no language-level safeguards against use-after-free.

The solution is not to provide the non-streaming API on the C layer at all. On the Rust side, the non-streaming API is a convenience API built on top of the streaming API and some validation functions (ASCII validation, UTF-8 validation, ISO-2022-JP ASCII state validation). Instead of trying to provide FFI bindings for the non-streaming API in an inconvenient manner, a similar non-streaming API can be recreated in C++ on top of the streaming API and the validation functions that were suitable for FFI.

While the C++ type system could represent the same kind of structure as Rust’s Cow<'a, str> e.g. as std::variant<std::string_view, std::string>, such a C++ Cow would be unsafe, because the lifetime 'a would not be enforced by C++. While a std::string_view (or gsl::span) is (mostly) OK as an argument in C++, as a return type it’s use-after-free waiting to happen. As with C, at best there would be some documentation saying that the output std::string_view is valid for as long as the input gsl::span is valid.

To avoid use-after-free risk, in the C++ API version that uses C++17 standard-library types, I simply ended up making the C++ decode_without_bom_handling_and_without_replacement() always copy and return a std::optional<std::string>.

In the case of Gecko though, it’s possible to do better while keeping things safe. Gecko uses XPCOM strings, which provide a variety of storage options, notably: dependent strings that (unsafely) borrow storage owned by someone else, auto strings that store short strings in an inline buffer and shared strings that point to heap-allocated reference-counted buffer.

In the case where the buffer to decode is in an XPCOM string that points to a reference-counted heap-allocated buffer and we are decoding to UTF-8 (as opposed to UTF-16), in the cases where we’d borrow in Rust (expect for BOM removal cases), we can instead make the output string point the same reference-counted heap-allocated buffer that the input points to (and increment the reference count). This is indeed what the non-streaming API for mozilla::Encoding does.

Compared to Rust, there is a limitation beyond the input string having to use reference-counted storage for the copy avoidance to work: The input must not have the UTF-8 BOM in the cases where the BOM is removed. While Rust can borrow a subslice of the input excluding the BOM, with XPCOM strings just incrementing a reference count only works if the byte content of the input and output is the entirely the same. When the first three bytes need to be omitted, it’s not the entirely the same.

While the C++ API version that uses C++17 standard library types builds the non-streaming API on top of the streaming API in C++, for added safety, the non-streaming part of mozilla::Encoding is not actually built on the streaming C++ API in C++ but built on top of the streaming Rust API in Rust. In Gecko, we have Rust bindings for XPCOM strings, so it’s possible to manipulate XPCOM strings from Rust.

Epilog: Do We Really Need to Hold Decoder and Encoder by Pointer?

Apart from having to copy in the non-streaming API due to C++ not having a safe mechanism for borrows, it’s a bit disappointing that instantiating Decoder and Encoder from C++ involves a heap allocation while Rust callers get to allocate these types on the stack. Can we get rid of the heap allocation for C++ users of the API?

The answer is that we could, but to do it properly we’d end up with the complexity of making the C++ build system generate constants by querying them from rustc.

We can’t return a non-C-like struct over the FFI by value, but given a suitably-aligned pointer to enough memory, we can write a non-C-like struct to memory provided by the other side of the FFI. In fact, the API supports this as an optimization of instantiating a new Decoder into a heap allocation made by Rust previously:

Even though documentation says that encoding_new_decoder_into() should only be used with pointers to Decoder previously obtained from the API, in the case of Decoder, assigning with = would be OK even if the memory pointed to by the pointer was uninitialized, because Decoder does not implement Drop. That is, in C++ terms, Decoder in Rust does not have a destructor, so assignment with = does not do any clean-up with the assumption that the pointer points to a previous valid Decoder.

When writing a Rust struct that implements Drop into uninitialized memory, std::ptr::write() should be used instead of =. std::ptr::write() “overwrites a memory location with the given value without reading or dropping the old value”. Perhaps it would set a good example to use std::ptr::write() even in the above case, even though it’s not strictly necessary.

When working with a pointer previously obtained from Rust Box, the pointer is aligned correctly and points to a sufficiently large piece of memory. If C++ is to allocate stack memory for Rust code to write into, we need to make the C++ code use the right size and alignment. The issue of communicating these two numbers from Rust to C++ is already where things start getting brittle.

The C++ code needs to discover the right size and alignment for the struct. These cannot be discovered by calling FFI functions, because C++ needs to know them at compile time. Size and alignment aren’t just constants that could be written manually in a header file once and forgotten. First of all, they change when the Rust structs change, so just writing them down has the risk of the written-down values getting out of sync with the real requirements as the Rust code changes. Second, the values differ on 32-bit architectures vs. 64-bit architectures. Third, and this is the worst, the alignment can differ from one 32-bit architecture to another. Specifically, the alignment of f64 is 8 on most targets, like ARM, MIPS and PowerPC, but the alignment of f64 is 4 on x86. If Rust gets an m68k port, even more variety of alignments across 32-bit platforms is to be expected.

It seems that the only way to get this right is to get the size and alignment information from rustc as part of the build process before the C++ code is built so that the numbers can be written in a generated C++ header file that the C++ code can then refer to. The simple way to do this would be to have the build system compile and run a tiny Rust program that prints out a C++ header with numbers obtained using std::mem::size_of and std::mem::align_of. This solution assumes that the build system runs on the architecture that the compilation is targeting, so this solution would break cross-compilation. That’s not good.

We need to extract target-specific size and alignment from a given struct from rustc but without having to run a binary built for the target. It turns out that rustc has a command-line option, -Zprint-type-sizes, that prints out the size and alignment of types. Unfortunately, the feature is nightly-only… Anyway, the most correct way to go about this would be to have a build script controlling C++ compilation first invoke rustc with that option, parse out the sizes and aligments of interest, and generate a C++ header file with the numbers as constants.

Or, since overaligning is permitted, we could trust that the struct will not have a SIMD member (alignment 16 for 128-bit vectors) and always align to 8. We could also check the size on 64-bit platforms, always use that and hope for the best (especially hope that whenever the struct grows in Rust, someone remembers to update the C++-visible size). But hoping for the best in memory matters kind of defeats the point of using Rust.

Anyway, assuming that we have constants DECODER_SIZE and DECODER_ALIGNMENT available to C++ somehow, we can do this:

Note that outside the implementation of Encoder trying to just declare Decoder decoder; without initializing it right away is a compile-time error, because the constructor Decoder() is private.

Let’s unpack what’s happening:

The array of unsigned char provides storage for the Rust Decoder.

The C++ Decoder has no base class, virtual methods, etc., so there are no implementation-supplied hidden members and the address of a Decoder is the same as the address of its storage member, so we can simply pass the address of Decoder itself to Rust.

The alignment of unsigned char is 1, i.e. unrestricted, so alignas on the Decoder gets to determine the alignment.

The default trivial move constructor memmoves the bytes of the Decoder, and the Rust Decoder is OK to move.

The private default no-argument constructor makes it a compile error to try to declare a not-immediately-initialized instance of the C++ Decoder outside the implementation of Encoder.

Encoder, however, can instantiate an uninitialized Decoder and pass a pointer to it to Rust, so that Rust code can write the Rust Decoder instance into the C++-provided memory via the pointer.

Written by Henri Sivonen, published 2018-12-03; update about unwinding across FFI added and typo fixed 2018-12-05

Code extracts from encoding_rs and encoding_c are Copyright Mozilla Foundation and are licensed under the Apache License, Version 2.0 <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.